Method for cooling a product, particularly, for liquefying a gas, and device for implementing this method

ABSTRACT

A method for cooling a product (P) including N ordered adsorption/desorption cycles ( 100, 200, 300 ), each cycle having the following steps: expanding a refrigerant in liquid phase from a condenser ( 101, 201, 301 ) inside an evaporator ( 103, 203, 303 ) for evaporating at least one portion of the refrigerant, and; adsorbing this refrigerant in vapor phase inside at least one adsorption/desorption chamber ( 120, 220, 320 ) containing a zeolite adsorbent (Z) whereby cooling a remaining portion of the refrigerant in the evaporator to a predetermined low temperature, the low temperature decreasing from one cycle to the next. The method also includes the following steps: effecting N-1 heat exchanges each time the refrigerant enters the evaporator ( 103, 203 ) of a cycle and each time the refrigerant enters the condenser ( 201, 301 ) of the following cycle for condensing the refrigerant in the condenser.

The present invention relates to a method for cooling a product and to adevice for implementing it. Such a method can be implemented inparticular for liquefying a product such as natural gas.

FR 2 489 101 discloses a cooling method and a device for implementing itwhich call upon the properties of the zeolite/water pair. However, theteachings of that document do not allow very low temperatures to beachieved.

Methods for liquefying a product in the vapor phase, in which, startingout from an initial state in the vapor phase, close to normaltemperature and pressure conditions, the product is subjected toisothermal compression by as much as a few tens or even a few hundredbar, then to isobaric cooling, and finally to isenthalpic expansion inorder to attain a final state in the liquid phase that is unstable atnormal, namely atmospheric, pressure, are known. A method such as thisis represented by curve B in FIG. 3, the initial state being indicatedby the point A. Such methods, for example the CLAUDE and LINDE methods,in particular allow the constituent parts of dehydrated air to beliquefied and separated. To do that, large compressors are needed, andthis leads to a significant expenditure in electrical or mechanicalenergy, that is to say secondary energy. Such a liquefaction method isan open conversion applied directly to the fluid product that is to becooled and therefore does not require any separate refrigerating fluid.Disadvantages are the energy expenditure needed for compressing theproduct to a supercritical state and the low proportion of the productthat contributes to the wanted effect (a liquid phase). Such a methodallows large temperature differences to be achieved at the expense of agreat deal of energy expenditure.

U.S. Pat. No. 5,339,649 describes a two-stage cryogenic refrigerator forliquefying helium intended to cool a superconductor magnet. In thecoldest stage, the refrigerating fluid is helium which is desorbed at apressure of between 14 and 18 atm by heating the adsorbent. Thissupercritical helium is not liquefied until downstream of theJoule-Thomson expansion valve. In the other stage, the refrigeratingfluid is hydrogen and a chemical absorbent such as LaNi₅ is used. Hereagain, the hydrogen is not liquefied until downstream of theJoule-Thomson expansion valve. Each time, there is only a fraction ofthe fluid to be liquefied and which therefore contributes to therefrigerating effect. The isochoric compression of the fluid in theadsorption chamber has the advantage of using up heat rather thansecondary energy. However, isochoric compression is more expensive inenergy than any other form of compression (isothermal, adiabatic). Theobtaining of helium and of supercritical hydrogen thus entails a verygreat expenditure in energy, proportional on the whole to the pressuredifference, and this weighs heavily on the energy efficiency of thismachine. In addition, the Joule-Thomson expansion of hydrogen does notallow the hydrogen to be liquefied unless sufficient precooling has beenachieved. This precooling is achieved by exchange of heat with a reserveof liquid nitrogen. The consumption of liquid nitrogen also weighsheavily on the energy efficiency of this refrigerator. This refrigeratoris a low-power small-sized machine designed to be installed onboard avehicle. Its low efficiency, which stems from the low fraction of fluidactually liquefied, from the cost of compression and from theconsumption of liquid nitrogen as a hot source to which the heat istransferred, makes it ill-suited to higher-powered applications.

The invention aims to provide a cooling method which can be applied inparticular to the liquefaction of a gas, which is less expensive interms of energy and which makes it possible to obtain a wide range offinal temperatures, particularly very low temperatures. Another objectof the invention is to provide a method and a device which are able toproduce coldness with high power and good energy efficiency,particularly in the temperature range between −80° C. and −220° C. Yetanother object is to propose an installation that allows the temperatureto be lowered to the desired temperature while at the same timecomplying with constraints concerned with the mass of the plant, thecost of cooling in terms of energy, and the dependability andreliability of the plant.

To do that, the invention provides a method for cooling a productcomprising N ordered adsorption/desorption cycles performed undervacuum, N being an integer greater than 1, each cycle comprising thesteps consisting in:

-   -   extracting heat from a refrigerating fluid in the vapor phase in        a condenser at a first pressure below the critical pressure of        said fluid for condensing said refrigerating fluid,    -   introducing said refrigerating fluid in the liquid phase into an        evaporator at a second pressure lower than the first pressure in        order to vaporize some of said refrigerating fluid and cool the        rest of said refrigerating fluid to a vaporization temperature        of said refrigerating fluid at said second pressure, said        vaporization temperature decreasing from one cycle to the next,        said first and second pressures being chosen in each cycle so        that said vaporization temperature in one cycle is each time        lower than the condensation temperature of the refrigeration        fluid in the next cycle at the first pressure of said next        cycle,    -   supplying heat to the liquid fraction of the refrigerating fluid        at said second pressure in said evaporator in order to evaporate        said refrigerating fluid,    -   adsorbing said refrigerating fluid in the vapor phase in at        least one adsorption/desorption chamber connected to said        evaporator and containing a zeolite adsorbent,    -   once a quantity of said refrigerating fluid has been adsorbed        into said zeolite adsorbent, regenerating said zeolite adsorbent        by heating in order to desorb said quantity of refrigerating        fluid in the vapor phase,    -   returning said quantity of refrigerating fluid in the vapor        phase to said condenser, said method further comprising the        steps consisting in: performing N-1 heat exchanges, each        performed between the refrigerating fluid in the evaporator of        one cycle and the refrigerating fluid in the condenser of the        next cycle in the order of the cycles in order thus to supply        said heat to said evaporator and extract said heat in said        condenser, and cooling said product by exchange of heat with the        refrigerating fluid at least in the evaporator of the last        cycle.

The zeolite is an adsorbent clay which has the ability to fix numerousbodies and which is available at low cost. For example, at normal orambient temperature it is able to fix water up to more than 25% of itsown weight. The adsorption is exothermal and the desorption isendothermal. During adsorption of water, about 3500 kJ are released perkg of water fixed. The zeolite can also be sized to act like a molecularsieve, so as to select which bodies to adsorb against a molecular-sizecriterion.

In each cycle, adsorption of the vapor phase in theadsorption/desorption chamber acts like a pumping action which lowersthe partial pressure of the refrigerating fluid and thus shifts thephase equilibrium in the evaporator so as to sustain the vaporization ofthe refrigerating fluid, and this cools the evaporator through theextraction of latent heat of vaporization. This pumping action isobtained in a physico-chemical manner without supplying mechanical work.The method thus uses only a small amount of secondary energy, in orderto circulate the refrigerating fluid. The latent heat removed in theevaporator of one cycle is compensated for on each occasion by anextraction of heat in the condenser of the next cycle, allowing therefrigerating fluid to be condensed in the condenser. In addition, thisextraction of latent heat is used, at least in the last cycle, to coolthe target product.

Regeneration consists in heating the adsorbent to reduce itsadsorbtivity and thus desorb the fluid. The regeneration temperature forthe zeolite may be chosen in each cycle so as to cause total or almosttotal desorption of the corresponding refrigerating fluid. However, aszeolite is a very poor conductor of heat, total desorption takes a longtime. For preference, desorption is therefore performed partially, forexample down to a content of 10% by weight, so as to accelerate thedynamics of the method.

Thus, the refrigerating fluid in each cycle is recirculated and cancirculate in a closed circuit for a long time. Most of the energyconsumed by the method, namely for regenerating the adsorbent, can beprovided in the form of heat, that is to say in the form of primaryenergy.

Advantageously, a separate refrigerating fluid is used in each cycle,each refrigerating fluid being selected so that it exhibits avaporization temperature at the second pressure of the correspondingcycle which is below the condensation temperature at the first pressureof the fluid used in the previous cycle, so that the transfer of heatfrom the fluid that is to be condensed to the fluid that is to beevaporated can occur. The criteria against which the fluids are selectedare, in each instance, the intrinsic properties of the fluid: latentheat of change of state, liquid-vapor then solid-vapor equilibriumcurve, critical temperature, triple point temperature, compatibilitywith the containment material, potential risks (explosion, toxicity),and the properties of the fluid/zeolite adsorbent pair: adsorption curve(rate of adsorption as a function of temperature), stability of thefluid in the presence of the zeolite. Having eliminated fluids that maypresent unacceptable risks, those which will allow the cascade to workare sorted according to the principle whereby evaporation in one stageis performed as a result of condensation in the next stage.

The refrigerating fluids are also chosen on the basis of the ability ofthe zeolite to perform significant and effective adsorption and on thebasis of the corresponding heat adsorption. The expected adsorptionrates may range up to 30% (for example 30% by mass of water in the caseof zeolite 13X, 20% by mass of water for zeolite 4A). The desorptiontemperatures can vary (250° C. for water on zeolite 4A, 70° C. fornitrogen on zeolite 4A, 90° C. for water on zeolite 13X). In all cases,adsorption is high at low temperature. The heats of adsorption are ofthe order of magnitude of 1.5 times the latent heat of vaporization ofthe adsorbed fluid.

According to some particular embodiments of the invention, saidrefrigerating fluids are chosen from water (Tb=100° C.), butane(Tb=−0.5° C.), ammonia (Tb=−33° C.), carbon dioxide (Tb=−37° C.),propane (Tb=−42° C.), acetylene (Tb=−84° C.), ethane (Tb=−88° C.),ethylene (Tb=−103.9° C.), xenon (Tb=−108° C.), krypton (Tb=−152° C.),methane (Tb=−161.6° C.), argon (Tb=−185° C.), nitrogen (Tb=−195.5° C.)and neon (Tb=−245.92° C.), where Tb denotes the boiling point at normalpressure. All these fluids, or at least some of them, can thus be used,in this order or in some other order, in the successive cycles. Forexample, the refrigerating fluid in the first cycle is water.

Advantageously, in at least one of said cycles, preferably in all thecycles, said refrigerating fluid has a latent heat of vaporizationhigher than 300 kJ/kg, preferably greater than or equal to about 450kJ/kg. The higher the exchanges of energy need to be, the moreappropriate it is to use fluids with a high latent heat. A minimumthreshold value of 300 kJ/kg for a fluid in the sequence of fluids, fora general value of around 450 kJ/kg for example is reasonable in amethane liquefaction plant. For smaller-sized plants that allowtemperatures to be reduced to lower values, this threshold may belowered.

Advantageously, in at least one of said cycles, preferably in all thecycles, the temperature in the evaporator is above the triple point ofsaid refrigerating fluid. Thus, a liquid phase rather than a solid phaseis obtained in the evaporator, allowing for more effective heatexchanges. In other words, the critical pressure of the fluid needs tobe higher than the first pressure (the high pressure) of the cycle, andthe triple point temperature needs if possible to be below the lowtemperature of the fluid, corresponding to the second pressure. However,the latter technical constraint may be lifted depending on the design ofthe exchangers.

As a preference, in at least one of said cycles, preferably in all saidcycles, the first pressure in said condenser is lower than 3 bar, forexample between 0.4 and 3 bar, and preferably close to normal pressure.Thus, the expenditure of energy needed to compress the refrigeratingfluid to the first pressure is lower.

The temperature in each condenser is, on each occasion, the condensationtemperature of the corresponding fluid at the first pressure obtainingin the condenser which may be normal pressure or some other pressure.During desorption, it is possible to compress a fluid to a comfortablepressure above normal pressure, for example to 2 atm, in order toincrease its condensation temperature in such a way as to tailor it tothe evaporation temperature of the fluid in the evaporator of theprevious stage.

Advantageously, in at least one of said cycles, preferably in all saidcycles, the maximum pressure is lower than 5 bar, preferably lower than3 bar, and more preferably still close to normal pressure. The mass ofthe plant for implementing the method is very sensitive to the highpressure of each of the cycles. For reasons of mechanical strength ofthe chambers and of thermal inertia of the components, high pressuresneed to be limited as far as possible. Thus, it is not necessary tobuild a plant able to tolerate large pressure differences. The cost andthe dependability of the refrigeration plant are thus improved.

The reliability of the plant is dependent on the leaktightness of thechambers. It is therefore appropriate to avoid working at excessivelylow low pressures. A minimum absolute pressure of 0.5 kPa or higher is,for example, a reasonable value for each of the cycles.

For preference, in at least one of said cycles, said refrigerating fluidin the liquid phase is introduced in atomized form into the evaporator.Thus evaporation is speeded up and the refrigeration power is thereforeimproved.

For preference, said extraction of heat in the condenser of the firstcycle is performed by exchange of heat with an environmental fluid atambient temperature. This environmental fluid to which the heat ofcondensation of the first cycle is removed and which thereforeconstitutes the hot source with respect to the refrigeration machineemploying the above method, may for example be atmospheric air or waterfrom a river, a lake or the sea.

Advantageously, in at least one of said cycles, the heating of saidzeolite adsorbent to be regenerated is performed by exchange of heatwith an environmental fluid at ambient temperature. For example,regeneration is thus performed in the second cycle and in the nextcycles as appropriate.

Advantageously, the method according to the invention comprises the stepthat consists in performing at least one exchange of heat, preferably atleast N-1 exchanges of heat, each heat exchange being between saidzeolite adsorbent undergoing adsorption in an adsorption/desorptionchamber of one cycle and said zeolite adsorbent undergoing regenerationin an adsorption/desorption chamber of the next cycle. Thus, theexchanges of heat between the adsorption/desorption chamber of one cycleand the adsorption/desorption chamber of the next cycle allowregeneration to be performed without supplying heat from the outside,except in the case of the regeneration of the first cycle. However, evenfor the first cycle, that is to say the cycle at which the highest hightemperature is attained, the method may be designed to operate at arelatively low temperature, for example 250° C. in the case of thezeolite/water pair. As a result, it is easy to procure a hot source atthe regeneration temperature for the first cycle. The method may thusoperate in cogeneration with an industrial plant that produces heat aseffluent, such as a heat engine.

Adsorption is an exothermal reaction and the adsorbtivity of the zeolitedecreases as its temperature increases. For preference, in each cycle,it is anticipated for the zeolite adsorbent to be cooled in theadsorption/desorption chamber in which said refrigerating fluid isadsorbed. In this way, the adsorbent can be kept at a suitable operatingtemperature.

For preference, a step is provided that consists in performing at leastone exchange of heat, preferably N-1 exchanges of heat, each heatexchange being between the refrigerating fluid in the evaporator of onecycle and said zeolite adsorbent in the adsorption/desorption chamber ofthe next cycle undergoing adsorption, in order to cool said zeoliteadsorbent. Thus, the cooling of the zeolite undergoing adsorption isobtained without any additional energy consumption.

Advantageously, in each cycle, there are at least twoadsorption/desorption chambers, so that said adsorption of therefrigerating fluid can be performed in one of saidadsorption/desorption chambers while at the same time said regenerationof the zeolite adsorbent is being performed in another of saidadsorption/desorption chambers.

For preference, in each cycle, there are at least threeadsorption/desorption chambers so that a step of cooling the zeoliteadsorbent after regeneration can also be performed at the same time inyet another of said adsorption/desorption chambers. Thus, eachadsorption/desorption chamber carries out three steps in succession: anadsorption step, during which it is preferable to cool the adsorbent, aregeneration or desorption step, during which the adsorbent is heated,and a post-regeneration cooling step during which the adsorbent iscooled before adsorption is resumed.

For preference, there is provided the step that consists in performingat least one exchange of heat, preferably N-1 exchanges of heat, eachheat exchange being between the refrigerating fluid in the evaporator ofone cycle and said zeolite adsorbent in the adsorption/desorptionchamber of the next cycle undergoing post-regeneration cooling. Thus,the cooling of the zeolite after regeneration can be obtained withoutadditional energy consumption.

Advantageously, there is provided the step that consists, in at leastone of said cycles, preferably in each of said cycles, in cooling saidquantity of refrigerating fluid in the vapor phase by exchange of heatwith a source at ambient temperature before said quantity ofrefrigerating fluid is reintroduced into the condenser. Thus, therefrigerating fluid can be cooled in two steps in order to condense it:first of all by exchange of heat with a source at ambient temperature,then in the condenser by exchange of heat with the evaporator of theprevious cycle. This is particularly advantageous when the targetcondensation temperature is below ambient temperature. When the targetcondensation temperature is above ambient temperature, the exchange ofheat with a source at ambient temperature may be enough to obtain thedesired condensation.

Within the meaning of the invention, the expression “under vacuum” meansthat the cycles take place under a reduced partial pressure of air, thestrength of the vacuum varying according to the rate of transfer that isto be obtained. Advantageously, the partial pressure of air in eachcycle is less than about 1 kPa, preferably less than about 0.1 kPa. Todo that, a vacuum pump is preferably provided in each stage, because ofthe imperfect leaktightness. It is also advantageous to provide a vacuumpump connected to said or each adsorption/desorption chamber, in orderto remove from the chamber any air and/or non-adsorbable impurities thatwere initially dissolved in the refrigerating fluid.

The low temperature of the last cycle is chosen according to theapplication. For example, it may range between −40° C. and −220° C. Avery low temperature is suitable in particular for liquefying certaingases.

Advantageously, the product to be cooled is initially in the vapor phaseand said product is cooled, particularly more or less isobarically, inorder to liquefy it. This method allows a body, for example, methane orthe constituent components of air, to be liquefied without using a highpressure, something which presents advantages in terms of equipment costand dependability.

The product to be cooled may be of any kind. According to one particularembodiment of the invention, said product is a gas used as a fuel or asa polymerizable raw material, for example liquefied petroleum gas,methane, ethane, propane, butane, ethylene, propylene, hydrogen and thelike, particularly so that said product can be loaded on board a liquidfuel gas transport ship or for land-based plants.

According to another particular embodiment of the invention, the productis a gas for use as a raw material, for example liquid air, nitrogen andoxygen, that is cooled or liquefied to between −80° C. and −220° C.

In an embodiment variant, a step is provided that consists in performingat least one exchange of heat, preferably N-1 exchanges of heat, eachheat exchange being between the refrigerating fluid in the condenser ofone cycle and said zeolite adsorbent at said high temperature of thenext cycle, said intermediate temperature of one cycle each time beinghigher than or equal to said high temperature of the next cycle. In thiscase, the high temperature and the intermediate temperature are alsochosen to decrease from one cycle to the next. However, it is necessaryto have good control over the conditions of condensation of therefrigerating fluid in the condenser, particularly over the temperature,in order to be able to use the heat of condensation according to thisvariant.

The invention also provides a device for implementing the aforementionedmethod, comprising N ordered cooling stages performed under vacuum, Nbeing an integer greater than 1, each stage comprising:

a condenser able to contain a refrigerating fluid in a liquid phase,

an evaporator connected to said condenser by a pipe,

at least one adsorption/desorption chamber containing a zeoliteadsorbent and connected to said evaporator via an upstream valve,

a pipe equipped with a downstream valve for returning the refrigeratingfluid from said adsorption/desorption chamber to said condenser,

a heating means in said or each adsorption/desorption chamber able toheat said zeolite adsorbent to a regeneration temperature, said devicecomprising N-1 heat exchangers each arranged in such a way as toexchange heat between the refrigerating fluid in the evaporator of onestage and the refrigerating fluid in the condenser of the next stage inthe order of the cycles in order to cool this fluid, and a final heatexchanger arranged in such a way as to exchange heat between a productthat is to be cooled and the refrigerating fluid in at least theevaporator of the last stage.

For preference, in at least one of said stages, a cooling chamber forcooling the refrigerating fluid is arranged between said or eachadsorption/desorption chamber and said condenser and is in thermalcontact with a source of heat at ambient temperature.

Advantageously, the device according to the invention comprises, by wayof heating means for heating said adsorption/desorption chambers, atleast one heat exchanger, preferably at least N-1 heat exchangers, eacharranged in such a way as to exchange heat between said zeoliteadsorbent undergoing adsorption in said or one of saidadsorption/desorption chamber(s) of one stage and said zeolite adsorbentundergoing regeneration in said or one of said adsorption/desorptionchamber(s) of the next stage.

Advantageously, a cooling means is provided in said or eachadsorption/desorption chamber in order to cool said zeolite adsorbentundergoing adsorption.

Advantageously, the device comprises, by way of cooling means forcooling said adsorption/desorption chambers, at least N-1 heatexchangers each arranged in such a way as to exchange heat between therefrigerating fluid in the evaporator of one stage and said zeoliteadsorbent in said or each adsorption/desorption chamber of the nextstage.

For preference, each stage comprises at least two, preferably three,adsorption/desorption chambers each connected to said evaporator via arespective upstream valve and to said condenser via a respectivedownstream valve. Thus, the device can operate continuously, adsorptionbeing carried out as a concurrent operation in each chamber insuccession while the other chambers are respectively undergoingregeneration and post-regeneration cooling.

Advantageously, the device comprises a means of controlling said valveswhich is programmed to open and close said upstream and downstreamvalves in a cycle of concurrent operations, in which each chamberperforms in succession an adsorption step, for which the upstream valveis open and the downstream valve is closed, a regeneration or desorptionstep for which the downstream valve is open and the upstream valve isclosed, and a post-regeneration cooling step for which the downstreamvalve and the upstream valve are closed.

According to an embodiment variant, provided that the conditions ofcondensation of the fluid in the condenser are well controlled, it ispossible to provide, by way of heating means for heating saidadsorption/desorption chambers, at least one heat exchanger, preferablyat least N-1 heat exchangers, each designed to exchange heat between therefrigerating fluid in the condenser of one stage and said zeoliteadsorbent in said or each adsorption/desorption chamber(s) of the nextstage.

According to one particular embodiment of the invention, the device isassociated with a chamber containing said product that is to be cooled,said final heat exchanger being supported within said chamber in orderto exchange heat between the refrigerating fluid in the evaporator ofthe last stage and the product in the liquid or vapor phase contained insaid chamber.

The invention also provides a methane tanker equipped with a storagetank for liquefied gas, with which a device as embodied hereinabove isassociated by way of a refrigerating re-liquefaction unit.

The invention also provides a gas-liquefaction plant comprising acooling chamber for cooling the gas that is to be liquefied, whichchamber is associated with a device as embodied hereinabove.

The invention will be better understood and other objects, details,features and advantages thereof will become more clearly apparent in thecourse of the following description of some particular embodiments ofthe invention which are given solely by way of nonlimiting illustrationwith reference to the attached drawings. In these drawings:

FIG. 1 is a diagrammatic depiction of a multi-stage cooling machine withthree stages for implementing the method according to a first embodimentof the invention;

FIG. 2 depicts, in greater detail, the first stage and part of thesecond stage of the machine according to a second embodiment of theinvention;

FIG. 3 is a thermodynamic diagram for nitrogen N₂;

FIG. 4 is a general operating diagram for a refrigerating machine;

FIG. 5 depicts a typical example of the adsorption curve for a zeoliteadsorbent as a function of temperature;

FIG. 6 depicts a three-stage refrigerating machine according to a thirdembodiment of the invention;

FIG. 7 depicts a thermodynamic cycle performed by the refrigeratingfluid in each stage of the refrigerating machine of FIG. 6;

FIG. 8 is a thermodynamic diagram representing the temperature andpressure conditions in the three stages of the machine of FIG. 6;

FIG. 9 is a schematic depiction of a plate-type heat exchanger;

FIGS. 10 to 12 depict embodiments of a device for breaking up a liquidjet.

With reference to FIG. 1 a multi-stage cooling machine comprising threestages numbered 100, 200 and 300 will now be described. Each stagecomprises a refrigerating fluid circuit of similar design and operation,in which a vacuum has been created and which will be described ingreater detail with reference to the first stage 100. In the nextstages, analogous elements bear the same reference numeral increased byone or two hundred.

In the first stage 100, the refrigerating fluid is water H₂O. Therefrigerating fluid circuit comprises a condenser 101 connected via pipe104 provided with a circulating pump 102 to an evaporator 103. Theevaporator 103 is connected via pipe 110 equipped with an upstream valve130 to an adsorption/desorption chamber 120. The chamber 120 contains ablock of zeolite Z by way of adsorbent. The adsorption/desorptionchamber 120 is connected via pipe 160 equipped with a downstream valve150 to the condenser 101. In the condenser 101, the water is in liquidphase at 1 atmosphere of pressure and at an intermediate temperature ofbelow 100° C., for example of around 80° C. The entire refrigeratingfluid circuit is under vacuum, for example with a partial pressure ofair of below 0.1 millibar (mbar). For that, each adsorption/desorptionchamber is connected to a vacuum pump which will be described withreference to FIG. 2. The partial pressures obtained may be monitoredusing a gas analyzer of known type.

Under the action of the circulating pump 102, the water in the liquidphase is injected through the pipe 104 into the evaporator 103 in whichit falls as a shower. The circulating pump 102 may be replaced by avalve when flow can be obtained under gravity. By being expanded in theevaporator 103 in a more or less adiabatic manner, the water partiallyvaporizes. As the upstream valve 130 is open, the water vapor passesfrom the evaporator 103 through the pipe 110 to theadsorption/desorption chamber 120 in which the water vapor is adsorbedby the block of zeolite Z. This adsorption reaction consumes the watervapor which appears in the evaporator 103, and this permanently sustainsthe vaporization of the water in order to compensate for the quantity ofvapor adsorbed. This continuous vaporization takes heat from the fluid,that is to say the water present in the evaporator 103, so that a lowtemperature, for example of between −10 and −30° C., obtains in theevaporator 103. A phase of solid ice is thus obtained in the bottom ofthe evaporator 103. The temperature and pressure obtaining in theevaporator 103 may be regulated through, on the one hand, the flow rateof vapor leaving through the upstream valve 130 and, on the other hand,the quantity of liquid water introduced from the condenser 101 and thequantity of heat introduced by the heat exchanger 280, which will bedescribed later on. Because of the phase equilibrium, the temperature inthe evaporator 103 is lower, the lower the pressure maintained. Inparticular, it might also be possible to obtain a liquid phase in theevaporator 103 by an appropriate choice of temperature and pressure.

The adsorption/desorption chamber 120 is equipped with a cooling meansused to cool the block of zeolite Z during the adsorption reaction,which is exothermal. Thus the temperature in the chamber 120 is kept atbelow 100° C. To do that, the cooling means is a circuit of heattransfer fluid 140 equipped with a circulating pump 194 and which is incommunication with a cold source 108 which may, for example, be water atambient temperature or alternatively may be the ambient atmosphere.

When the block of zeolite Z has adsorbed a certain quantity of water itneeds to be regenerated. To do that, the upstream valve 130 is closedand the downstream valve 150 is opened. Some three-way valves 124 arethen switched in order to connect the heat transfer fluid circuit 140 toa hot source 109 receiving heat H from outside. The hot source 109 maybe any heat source at a temperature preferably higher than 250° C. Thus,the heat transfer fluid circuit 140 acts as a heating means which heatsthe block of zeolite Z to, for example, 250° C. At this temperature, theadsorptivity of the zeolite is very low. Desorption is prolonged untilalmost 90% of the water vapor has been desorbed. As the thermalconductivity of zeolite drops as its water content is reduced, desorbingfurther water would take a great deal of time, which would slow down themethod and thus reduce the temporal efficiency of the machine. Under theeffect of the pressure in the chamber 120, the water vapor desorbed isremoved from the chamber 120 through the pipe 160, the downstream valve150 being open. The water vapor escapes into the condenser 101 in whichit is condensed and cooled to the intermediate temperature of 80 to 100°C. To do that, the condenser 101 is constantly cooled via a coolingmeans 126, for example a heat exchanger connected with atmospheric air.A fan 115 is provided to improve the cooling of the condenser 101.

In the evaporator 103, the vaporization reaction of the water is used toextract heat from a coil 125 through which a heat transfer fluidcirculates. The coil 125 belongs to a heat exchanger 280 which alsocomprises a coil 226 arranged in the condenser 201 of the second stage200 and a coil 270 arranged in the adsorption/desorption chamber 220 ofthe second stage 200. The heat exchanger 280 also comprises acirculating pump 227 which circulates the heat transfer fluid from theevaporator 103 into the condenser 201 so as to cool the condenser 201then into the adsorption chamber 220 so as to cool the block of zeoliteZ undergoing adsorption. The heat exchanger 380 performs the samefunction between the second stage 200 and the third stage 300.

In the second stage 200, the refrigerating fluid is, for example, butaneC₄H₁₀. The intermediate temperature in the condenser 201 is between −10and −20° C. at a pressure of 1 or 2 atmospheres, the butane thereforebeing in the liquid form. The temperature obtained in the evaporator 203is between −60 and −80° C. To regenerate the block of zeolite Z in theadsorption/desorption chamber 220, the temperature is raised to a hightemperature of 80° C. To do that, use is made of a heat transfer fluidcircuit 240 connected to the hot source 109 and/or some other heatingmeans which will be described with reference to FIG. 2.

Unlike the stage 100, the stage 200 comprises an intermediate coolingreservoir 216 between the pipe 260 which runs from the chamber 220 andthe condenser 201. This reservoir 216 is connected to the condenser 201via a valve 217. The reservoir 216 is lagged when the fluid desorbed isat a temperature below ambient temperature, to prevent an increase inpressure in the reservoir 216. The reservoir 216 is placed in thermalcontact with the ambient atmosphere when the fluid desorbed is at atemperature above ambient temperature, so as to obtain a first coolingof the butane in vapor phase after desorption and thus prevent anincrease in pressure in the reservoir 216. In the latter instance, thereservoir 216, which is, for example, a conventional steel gas storagecylinder, is positioned outside and a heat exchanger 214 ventilated by afan 215 is provided.

During regeneration, the block of zeolite Z in the chamber 220 isheated, for example to 80° C. Desorption of the butane causes a rise inpressure in the chamber 220 and therefore causes butane in vapor phaseto flow through the pipe 260 to the reservoir 216. When the butaneundergoes cooling, this cooling causes butane to be drawn from thechamber 220. In this step, the temperature in the reservoir 216 dependson the conditions under which desorption occurs, that is to say inparticular on the pressure in the reservoir 216.

For example, a pressure of 15 bar may be provided. The more the pressurein the reservoir 216 is allowed to rise, the greater the temperaturetherein will be. From the reservoir 216, the butane in vapor phase isthen expanded through the valve 217 in the condenser 216 in order to becooled further there and liquefied to a pressure close to normalpressure.

The stage 300 is similar to the stage 200. In the third stage 300, therefrigerating fluid is carbon dioxide CO₂. The intermediate temperaturein the condenser 301 is between −60 and −70° C., the carbon dioxidetherefore being in the liquid phase. The low temperature in theevaporator 303 is between −120 and −130° C. To regenerate the block ofzeolite Z in the adsorption/desorption chamber 320, the temperature israised to a high temperature between −10 and −20° C. To do that, use ismade of heating means of the same type as were used in the second stage200, namely a heat transfer fluid circuit 340 connected to the hotsource 109 or to a heat exchanger associated with anadsorption/desorption chamber of the previous stage, as will beexplained with reference to FIG. 2. During regeneration, in order forcooling at ambient temperature in the reservoir 316 to be effective, thetemperature in the reservoir 316 has of course to be above ambienttemperature, which assumes that a high enough pressure can be had in thereservoir 316.

Another option is to perform desorption at a lower pressure, so as toobtain a vapor temperature in the reservoir 316 (or 216) which is lowerthan ambient temperature. In this case, the exchanger 314 (or 214) isomitted. By contrast, in this case the reservoir 316 (or 216) is lagged.This variant may be preferred, according to the refrigerating fluidsused, each time the pressure in the reservoir that would make itpossible to obtain a temperature above ambient temperature is so highthat it gives rise to unacceptable technological constraints.

The evaporator of the last stage, that is to say the evaporator 303 ofthe third stage 300 in the exemplary embodiment depicted in FIG. 1, isprovided with means for using the coldness produced by the multi-stagemachine. For that, a final heat exchanger 80 is arranged between theevaporator 303 of the third stage 300 and a chamber 1 containing theproduct P that is to be cooled. The heat exchanger 80 comprises a heattransfer fluid circuit with a coil 325 in the evaporator 303 in whichthe heat transfer fluid is cooled and a coil 26 supported in the chamber1 and in which the heat transfer fluid is heated, cooling the product P.For example, the chamber 1 is a storage tank for storing a liquefied gasthat is to be cooled in order to compensate for thermal losses throughthe walls of the chamber 1. Depending on the application, it is alsopossible to use coldness taken from the evaporators of the other stagesof the machine, by providing corresponding heat exchangers.

Before the machine is started, in the initial state, the refrigeratingfluids are stored at ambient temperature in the reservoirs 101, 216 and316 respectively. The stages 100, 200 and 300 are started in succession.

As depicted in FIG. 1, the multi-stage machine cannot provide coolingduring the phase in which the blocks of zeolite Z are being regenerated.To remedy this disadvantage, with reference to FIG. 2, a secondembodiment of the machine is now described in which embodiment at leasttwo, and preferably three, adsorption/desorption chambers are providedin each stage. In FIG. 2, elements identical or similar to those of thefirst embodiment are denoted by the same reference numerals.

In FIG. 2, in the detailed depiction of the first stage 100, theadsorption/desorption chamber 120 and the corresponding upstream anddownstream valves are replaced by three adsorption/desorption chambers121, 122 and 123 which are connected respectively to the condenser 101via pipes 161 to 163 equipped with downstream bars 151 to 153. Thechambers 121 to 123 are also connected to the evaporator 103 via pipes111 to 113 equipped with upstream valves 131 to 133. The valves 131 to133 and 151 to 153 are electrically-operated valves controlled by acontrol unit 105 via control lines 107. The control unit 105 isprogrammed to perform a cycle of concurrent operations in which,simultaneously:

-   -   one chamber, for example 121, is undergoing adsorption, the        associated upstream valve being open and the associated        downstream valve being closed,    -   another chamber, for example 123, is undergoing regeneration,        the upstream valve being closed and the downstream valve being        open,    -   and the third chamber, for example 122, is undergoing cooling,        the upstream valve and the downstream valve being closed.

The valves are thus periodically switched in such a way that eachchamber performs the adsorption step, the regeneration step and thepost-regeneration cooling step in succession. The steps are notnecessarily of the same duration, which means that the changes in stepdo not necessarily occur simultaneously in all the chambers of onestage. In order to perform the cycle of concurrent operations, it ispreferable for the switching between chambers to be performedsynchronously in all the stages, for reasons of simplicity.

If just two chambers are provided, the regeneration step and the coolingstep in respect of one chamber are carried out during the adsorptionstep of the other chamber. In this way, at any moment, there is at leastone chamber undergoing adsorption in each stage.

Each chamber 121 to 123 is equipped with a cooling coil 171 to 173connected to a cold source and with a heating coil 141 to 143 connectedto a hot source. By circulating heat transfer fluids, the block ofzeolite Z is thus cooled during the adsorption step and during thepost-regeneration cooling step, and the block of zeolite is also cooledduring the regeneration step.

As a variant, as depicted in FIG. 1, with the numeral 140, a single heattransfer fluid circuit may be used as a cooling means and as a heatingmeans by connecting it selectively either to a hot source or to a coldsource. Because of the thermal inertia to which this gives rise, such asetup is, however, less advantageous.

A vacuum pump 106 is connected to each adsorption/desorption chamber 121to 123 in order to sustain the vacuum. The vacuum pump 106 serves tocompensate for lacks of leaktightness of the refrigerating fluid circuitand to draw out uncondensable bodies that were initially dissolved inthe water, for example oxygen, and which would carry the risk ofremaining in the adsorption chamber because they cannot be adsorbed bythe zeolite. After one or two cooling cycles, the water will be free ofsuch incondensables.

The heat exchanger 280 has been depicted in FIG. 2 as being connected tothe cooling coil 272 of the adsorption/desorption chamber 22. However,it is each of the adsorption chambers 221 to 223 that needs to be cooledduring the adsorption step and during the post-regeneration coolingstep. To achieve that, a separate heat exchanger may be provided foreach of the adsorption/desorption chambers 221 to 223 so as to exchangeheat with the evaporator 103. As a variant, the heat exchanger 280 willbe equipped with multi-way valves so that the cooling heat transferfluid can be circulated selectively through one or several cooling coils272 to 273. According to yet another variant, it is possible to provideat least two separate heat exchangers for exchanging heat, on the onehand, between the evaporator 103 and the condenser 201 and, on the otherhand, between the evaporator 103 and the adsorption chambers 221 to 223.

By way of a means of heating the blocks of zeolite Z in the second stage200 a heat exchanger 290 has been depicted which allows a heat transferfluid to be circulated through a cooling coil 171 of the chamber 121 ofthe first stage which is undergoing the adsorption step and in which theheat transfer fluid is heated to 80 to 100° C., then through the heatingcoil 243 of the adsorption/desorption chamber 223 which is undergoingthe regeneration step. Thus, each adsorption/desorption chamber of thesecond stage can be heated to 80° C. for its regeneration. Once again,it is each of the chambers 221 to 223 which needs, when performing itsown regeneration step, to exchange heat with the chamber 121 or 122 or123 simultaneously undergoing its own adsorption step. For that, severalheat exchangers similar to the exchanger 290 may be provided, oralternatively multi-way valves may be provided, these valves beingswitchable so that one or other of the cooling coils 171 to 173 can beconnected selectively to one or other of the heating coils 241 to 243.The heat transfer fluid is circulated through the heat exchanger 290 bymeans of a pump 294.

As a variant or as a supplement it is possible to provide a heatexchanger between the adsorption/desorption chambers 221 to 223 of thestage 200 and the condenser 101 of the stage 100 so as to use the heatof condensation of the water to regenerate the zeolite in the stage 200.However, it is necessary to have good control over the condensationtemperature in the condenser 101 in order to be able to proceed in thisway.

Heat exchangers similar to the exchangers 280 and 290 depicted in FIG. 2are connected between all the successive stages of the machine, suchthat each stage produces the coldness needed to condense therefrigerating fluid in the next stage, to cool the zeolite in theadsorption phase in the next stage, and produces the heat required toperform desorption of the refrigerant in the next stage, or at leastsome of this heat. Thus, in a preferred embodiment, only the stage 100requires an external supply of heat H by means of the hot source 109.However, it is also possible to use external heat H for regeneration inall the stages.

In FIG. 1, the machine comprises three stages mounted in cascade.However, it is possible to provide fewer or more stages with, in eachstage, a high temperature, an intermediate temperature and a lowtemperature chosen to be below the corresponding temperature in theprevious stage. For example, it is possible to provide a fourth stageusing ethylene as a refrigerating fluid, a fifth stage using methane asa refrigerating fluid, a sixth stage using nitrogen or argon as arefrigerating fluid, and a seventh stage using neon as a refrigeratingfluid. The low temperature in the last stage could then be close to 10K.

Table 1 gives thermodynamic properties of a certain number of bodiesthat can be used as refrigerating fluids in the multi-stage coolingmachine. The boiling point of each is given for several pressure valuesin order to illustrate the range of temperatures that can be obtained inthe evaporator for each stage, according to the choice of refrigeratingfluid. In the first stage, it is also possible to use a mixture of waterand glycol in order to obtain a higher melting point. Thus, it ispossible to avoid ice forming in the evaporator 103. The zeolite Z inthe chambers 121 to 123 will then be chosen in a size that prevents anyadsorption of the glycol, so that this glycol remains in the evaporator103.

Zeolite is available in numerous forms that differ from one another interms of their crystal structure or in terms of the pore size. Ingeneral, for each cycle, the form of zeolite best suited to therefrigerating fluid used is chosen.

In one particular embodiment, zeolite that has a particular structureallowing adsorption and desorption reactions to be accelerated ischosen. Such a structure is described in EP-A-470 886. Zeolite ingranular form, with a diameter of 3 mm for example, is deposited byimmersion on a metal component on which projections have been fashionedto delimit interstitial spaces. The zeolite fills these spaces and isfixed by sintering. With such a high-speed structure, one stage of themachine, for example the stage 100, can be run with just oneadsorption/desorption chamber and with the adsorption and desorptionsteps taking place in quick succession, for example once a minute.

The cooling machine described hereinabove has numerous applications. Forexample, it may be used as a cooling unit associated with a tank fortransporting liquefied gas in a methane tanker. For that the coil 26 ofthe final heat exchanger 80 will be suspended in the tank. In order toproduce such an arrangement, reference will be made to application FR 2785 034 A1. The temperature in the final heat exchanger 80 must be belowor equal to −164° C. in order to liquefy methane at normal pressure. Todo that, it is of course necessary to provide more stages than have beendepicted in FIG. 1, for example adding a stage using ethylene as arefrigerating fluid and a stage using methane as a refrigerating fluid.For this type of application, the use of oxygen as a refrigerating fluidis avoided, because of the risks of explosion in the event of leakage.

The multi-stage machine may also be used for carrying out isobariccooling of a gaseous body that needs to be liquefied in a liquefactionplant. To do that, the chamber 1 is used as a gas liquefaction chamber.The multi-stage machine may be applied to the liquefaction of air or itsconstituent components, including rare gases. The curve C in FIG. 3represents the thermodynamic path taken by the nitrogen in such aliquefaction method, from the initial state represented by the point A.

An analogous path may be plotted on the temperature-entropythermodynamic diagram for another gas, particularly methane. Suchdiagrams are available in the reference work “1′ encyclopédie des gaz[encyclopedia of gases]”, ISBN 0-444-41492-4 (1976-2002). For example,it is possible to obtain a liquefied methane flow rate of about 20kg/min with 1 t of zeolite in each adsorption/desorption chamber.

The heat transfer fluid circuits described hereinabove constitute merelyone illustrative example of a heat exchanger. Numerous other types ofexchanger may be used to implement the method according to theinvention, particularly without using an intermediate heat transferfluid.

Another embodiment of a refrigerating machine involving stages is nowdescribed with reference to FIG. 6. Elements identical or analogous tothose of the first embodiment bear the same reference number increasedby 300.

In each of the stages, a refrigerating fluid performs a thermodynamiccycle the principle of which is represented in FIG. 7, which is ageneral diagram that can be applied to various fluids.

In FIG. 7, the abscissa axis represents the specific entropy s and theordinates axis represents the temperature T. The line 37 represents theliquid/vapor phase change curve and the line 36 represent the criticalisobaric curve for the fluid.

Two isobaric curves corresponding to two pressures P₁<P₂ chosen betweenthe triple point and the critical pressure for the fluid have also beendepicted. The cycle is a closed cycle represented by the curve 38.

The fluid in liquid phase is evaporated isobarically at the pressure P₁and the temperature T₁ by placing it in communication with a zeoliteadsorbent that is kept at an adsorption temperature T_(ads) foradsorbing the vapor phase. After adsorption, the zeolite is heated to adesorption temperature T_(des)<T_(ads). The desorbed vapor undergoesisobaric compression to the pressure P₂. The fluid in the vapor phase iscondensed isobarically at the pressure P₂ and the temperature T₂.Finally, the pressure of the liquid phase is reduced sharply from P₂ toP₁, by means of a pressure drop, thus vaporizing some of the fluid.

In the successive stages, the fluids and the pressures are chosen sothat the evaporation of the fluid in a stage of rank n takes the heatnecessary for condensing the fluid from the next stage of rank n+1. Theoperating condition is therefore T₁(n)<T₂(n+1).

FIG. 5 depicts an example of an adsorption curve 39 showing the rate ofabsorption ι (as a percentage by mass of fluid adsorbed with respect tothe quantity of adsorbent) as a function of temperature T. Below theadsorption temperature T_(ads), the rate is more or less equal to thesaturation rate ι₀. To reduce the adsorption rate to a level ι₁<ι₀, thetemperature of the adsorbent has to be raised to a correspondingdesorption temperature T_(des). In practice, the curve 39 is dependentupon the fluid/adsorbent pair. In all the stages, the purpose of theadsorption reactors is to provide two vapor pressure levelscorresponding to two different temperature levels.

The three stages 400, 500 and 600, the structures and functioning ofwhich are similar, are described together. In the condenser 401 (or 501or 601) the fluid is condensed at the high pressure of the cycle Pithrough the extraction of the heat of condensation. The condensed fluidflows under gravity through a pipe 404 (or 504 or 604) to the evaporator403 (or 503 or 603) undergoing a pressure drop to the low pressure ofthe cycle P₂. Flow through the evaporator occurs naturally on eachoccasion. All that is required is for its flow rate to be regulated inorder to tailor the adsorptivities and evaporation capacities to suitthe temperature and pressure conditions desired in the evaporator. Theevaporator is lagged.

The condenser 501 (or 601) of the next stage in this instance isproduced in the form of hollow plates of a heat exchanger which arearranged in the evaporator 403 (or 503) and over which the liquidintroduced into the evaporator is made to trickle in ordersimultaneously to evaporate this liquid and condense the refrigeratingfluid in the condenser. FIG. 9 schematically depicts such a heatexchanger plate 25 on which a film of liquid 27 is undergoingevaporation.

In order to improve the evaporation dynamics, it is also possible todiffuse the liquid introduced into the evaporator using a jet break-updevice 435 (or 535 or 635) arranged at the end of the pipe 404 (or 504or 604). Devices such as this in several types, for example involving asingle orifice (FIG. 10), involving multiple orifices (FIG. 11), orinvolving a helix on a jet (FIG. 12), are known, these having beendeveloped, for example, for supplying fuel to burners. In this instance,they need to be heated in order to prevent them from becoming plugged.

A pipe 410 (or 510 or 610) connects the evaporator to three reactors421-423 (or 521-523 or 621-623) containing a zeolite absorbent. Shut-offvalves, not depicted (for example nonreturn valves), allow the reactorsto be isolated individually from the evaporator and from the condenser,so that, at any moment, one of the reactors at least is connected to theevaporator and cooled to an appropriate adsorption temperature T_(ads)so that vapor formed in the evaporator can be absorbed (exothermalreaction), and one of the reactors at least is connected to thecondenser and heated to a suitable desorption temperatureT_(des)>T_(ads) so as to release the vapor (endothermal reaction) to thecondenser at the high pressure of the cycle.

A high pressure P₁ that is not very high, for example lower than 5 barabsolute, or even lower than 3 bar absolute, is preferably chosen, so asto limit the wall thickness and thus the weight and cost of the machine.

If the condenser is likened to a spherical jacket of radius R, then thethickness e can actually be expressed as a function of the pressuredifference ΔP between the inside and outside, as follows:$e = \frac{\Delta\quad{PR}}{2\sigma_{adm}}$

For a cylindrical jacket of radius R, the factor of 2 is omitted.Considering a spherical chamber of radius 3 m, for a maximum permissiblestress σ_(adm)=240 MPa, without a factor of safety, the thickness is: ΔP(bar) 0.5 1 1.5 2 3 5 10 15 20 e (mm) 0.3 0.6 0.9 1.3 1.9 3.1 6.3 9.412.5 M (kg) 276 551 827 1103 1654 2757 5513 8270 11027

For this type of installation, a factor of safety of 5 seems the mostprobable. The mass will increase by the same factor.

EXAMPLE 1

A machine having up to 5 stages with the structure depicted in FIG. 6was produced. Table 2 gives, for each stage, a list of bodies that canbe used as refrigerating fluid, the high temperature T₂ attained in thecondenser and the low temperature T₁ attained in the evaporator.

The fluids are classified according to their equilibrium temperatures atpressures of 1 bar and 0.5 kPa.

EXAMPLE 2

A three-stage cooling machine with the structure depicted in FIG. 6 wasproduced. The parameters for each stage are given in table 3.

FIG. 8 provides greater detail about the cycles of the three fluids,with a representation similar to that of FIG. 7.

This machine can be used for cooling a product P to −150° C.approximately, for example by circulating this product through a heatexchanger 701 housed in the evaporator 603 of the last stage. To coolthe adsorption reactors 421-423 during the adsorption phase a heatexchanger 480 is provided that allows the heat to be removed to theatmospheric air or to a body of water at ambient temperature. Toregenerate these reactors, a heat source at 250° C. is provided, asdescribed above with reference to FIG. 1.

In the butane and ethane stages, the reactors are cooled in theadsorption phase by a heat exchanger 580 (or 680) for removing heat tothe evaporator 403 (or 503) of the previous stage. Although the heatexchangers 480, 580 and 680 have been depicted schematically, theycomprise the valves and configurations needed to allow each of reactorsto be cooled selectively.

In the butane and ethane stages there is also provided a heat exchanger540 (or 640) designed to exchange heat between the atmospheric air, theconvection of which is preferably forced by a fan 541 (or 641), and thezeolite adsorbent with a view to regenerating it at ambient temperatureor a temperature slightly higher than ambient temperature. If theregeneration temperature is higher than the ambient temperature, then inaddition heat may be exchanged with the condenser 401 of the water stagein order to provide the additional heat to the zeolite. Here again, thevalves and configurations that allow each of the reactors 521-523 (or621-623) to be heated selectively are provided. In this way, the supplyof external energy to the machine is only to the first stage, when thereactors 421-423 are being regenerated.

EXAMPLE 3

A machine for liquefying methane at normal pressure comprises fourstages the first three of which are similar to Example 2. Table 4 givesthe parameters for each stage. A stage in which the fluid is methane isadded, in accordance with the structure set out in FIG. 6.

On the basis of an adsorption rate of 10%, a cycle time of 1 hour foreach stage, an adsorption heat of one and a half times the latent heat,and three reactors per stage, the following adsorption and desorptiontemperatures may be anticipated: Fluid Water Butane Ethane MethaneT_(ads)  80° C. −30° C. −30° C. −30° C. T_(des) 180° C.   70° C.   70°C.   70° C.

In this configuration, the exchangers 580 and 680 can be modified insuch a way as to cool the reactors undergoing adsorption of the lowerstages by using the coldness produced in the evaporator 403 of the waterstage. If necessary, for that use may be made of the coldness producedin the evaporator 503 of the butane stage.

In addition, heat exchangers, not depicted, for regenerating thereactors 521-523, 621-623, etc. of the lower stages using the heat ofadsorption produced in the reactors 421-423 of the water stage, possiblycombined with the heat of condensation of the water which is released inthe condenser 401, are provided. Thus, only the reactors of the waterstage consume external energy for their regeneration, for example in theform of heat at 250° C.

EXAMPLE 4

The machine of Example 3 is sized so that it forms a plant forreliquefying the gas of evaporation loaded on board a methane tanker.

For a cargo volume of 125000 m³ of liquid methane and an evaporationrate of 0.15%/d, the useful refrigerating power required is estimated atabout 580 kW. For that, the following orders of magnitude areanticipated: Fluid Water Butane Ethane Methane Mass of adsorbent 13 t 18t 15 t 14 t Heat released 1.1 MJ 1.1 MJ 1.1 MJ

The total mass of the plant is of the order of 200 t and its dailyconsumption is 2.7 t/d of methane. These masses are proportional to thecycle time.

EXAMPLE 5

The machine of Example 3 is sized to load a methane tanker with aliquefied gas containing a high methane content in 24 h starting outfrom gases at 30° C. for a cargo volume of 125000 m³. The usefulrefrigerating power required is estimated at 630 MW. For that, thefollowing masses of adsorbent are anticipated: Fluid Water Butane EthaneMethane Mass of adsorbent 14 157 t 19 602 t 16 335 t 15 246 t

The total mass of the plant is of the order of 217800 t and its dailyconsumption is 2940 t/d of methane.

FIG. 4 is a general operating diagram for a refrigerating machine M. Inoperation, the refrigerating machine M transfers heat Q from a coldsource to a hot source N the temperature T_(ch) of which is higher thanthat of the cold source T_(fr). To do that, the machine M consumesenergy E. The efficiency of the machine is defined by the ratio Q/E. Inthe embodiments described hereinabove, the cold source consists of aproduct P that is to be cooled and the hot source preferably consists ofthe ambient atmosphere or the sea.

One way of favorably affecting this efficiency is to limit thecompression work by limiting the high pressure of the cycles, that is tosay the condensation pressure, to a value lower than a few bar, such asin Examples 2 and 3. In addition, given the pressure difference, limitedto about 1 bar, in each stage, the vapor obtained during regeneration ofthe zeolite is slightly superheated and does not require anysupercooling, because isochoric compression by 1 bar produces a heatingeffect of just 60° C. approximately.

In applications involving a high refrigeration power it is preferable touse fluids with a high latent heat of vaporization/condensation, so asto limit the refrigerating fluid flow rates and the mass of theadsorbents. Table 5 gives physical properties of various bodies that canbe used as refrigerating fluids in the methods according to theinvention: critical pressure Pc, critical temperature Tc, triple pointtemperature and latent heat L, which is a parameter used for classifyingthe fluids in this table.

Although the invention has been described in conjunction with severalspecific embodiments, it is quite obvious that it is not in any wayrestricted thereto and that it comprises all technical equivalents ofthe means described and combinations thereof where these fall within thescope of the invention. TABLE 1 Boiling points (° C.) of several bodiesthat can be used as refrigerating fluids at various pressures P = 1 atmP = 2 atm P = 1 Torr P = 10 Torr (1.013 × 10⁵ (2.026 × 10⁵ Body (1.33 ×10² Pa) (1.33 × 10³ Pa) Pa) Pa) Ammonia −109 −92 −33 −18.7 NH₃ Butane−101 −78 −0.5 −18.8 C₄H₁₀ Methane −205 −195 −164 −152 CH₄ Nitrogen −226−219 −196 −189 N₂ Carbon −134 −119 −78 −69 dioxide CO₂ Propane −129 −108−42 −25 C₃H₈ Neon Ne −257 −254 −246 −243 Xenon Xe −168 −108 Argon Ar−218 −210 −185 −179 Acetylene −143 −128 −84 −71 C₂H₂ Ethylene −168 −153−104 C₂H₄ Krypton −199 −187 −152 Kr Ethane −183 −159 −88 −75 C₂H₆

TABLE 2 Stage Fluid T₂ T₁ 1^(st) Water, alcohols ambient Down to −40° C.2^(nd) Butane, butadiene,  −20° C.  −80° C. propadiene, propane 3^(rd)Ethane, CO₂  −80° C. −150° C. Nitrous oxide 4^(th) Methane, krypton−150° C. −200° C. 5^(th) Neon, oxygen, −200° C. −260° C. helium,nitrogen, argon, CO

TABLE 3 Stage Fluid P₁ T₁ P₂ T₂ 400 Water 0.1032 kPa −20° C.  7.3 kPa  40° C. 500 Butane    1 kPa −81° C.  45 kPa −20° C. 600 Ethane   0.1kPa −153° C.  150 kPa −81° C.

TABLE 4 Fluid Water n-butane Ethane Methane P₂(bar) 0.8 1 1 3 T₂(° C.)80 −5 −88 −148 P₁(kPa) 0.5 0.5 0.5 20 T₁(° C.) −30 −88 −155 −182

TABLE 5 Triple Pc (bar) Tc (° C.) (° C.) L (kJ/kg) Water 221 376 0 2500CO₂ 74 31 −56.6 571 Methane 46 −82.2 −182.5 510 Ethane 49 32.2 −183.3489 Ethylene 50 9.5 −169 482 Hydrogen 13 −240 −259.3 454.3 Propane 42.596.6 −187.7 425.3 Butane 38 152 −125 386 Nitrogen 34 −147 −210 198Krypton 55 −63.8 −157 107.8 Xenon 58.4 16.5 −111.8 96.29 Neon 27.56−228.8 −249 88.7 Helium 2.2 −268 −272.2 20

1. A method for cooling a product (P) comprising N orderedadsorption/desorption cycles (100, 200, 300, 400, 500, 600) performedunder vacuum, N being an integer greater than 1, each cycle comprisingthe steps consisting in: extracting heat from a refrigerating fluid inthe vapor phase in a condenser (101, 201, 301, 401, 501, 601) at a firstpressure (P₂) below the critical pressure of said fluid for condensingsaid refrigerating fluid, introducing said refrigerating fluid in theliquid phase into an evaporator (103, 203, 303, 403, 503, 603) at asecond pressure (P₁) lower than the first pressure in order to vaporizesome of said refrigerating fluid and cool the rest of said refrigeratingfluid to a vaporization temperature (T₁) of said refrigerating fluid atsaid second pressure, said vaporization temperature decreasing from onecycle to the next, said first and second pressures being chosen in eachcycle so that said vaporization temperature (T₁) in one cycle is eachtime lower than the condensation temperature (T₂) of the refrigerationfluid in the next cycle at the first pressure of said next cycle,supplying heat to the liquid fraction of said refrigerating fluid atsaid second pressure in said evaporator in order to evaporate saidrefrigerating fluid, adsorbing said refrigerating fluid in the vaporphase in at least one adsorption/desorption chamber (120, 220, 320,421-423, 521-523, 621-623) connected to said evaporator and containing azeolite adsorbent (Z), once a quantity of said refrigerating fluid hasbeen adsorbed into said zeolite adsorbent, regenerating said zeoliteadsorbent by heating in order to desorb said quantity of refrigeratingfluid in the vapor phase, returning said quantity of refrigerating fluidin the vapor phase to said condenser, said method further comprising thesteps consisting in: performing N-1 heat exchanges, each performedbetween the refrigerating fluid in the evaporator (103, 203, 403, 503)of one cycle and the refrigerating fluid in the condenser (201, 301,501, 601) of the next cycle in the order of the cycles in order thus tosupply said heat to said evaporator and extract said heat in saidcondenser, and cooling said product by exchange of heat with therefrigerating fluid at least in the evaporator (303, 603) of the lastcycle.
 2. The method as claimed in claim 1, characterized in that saidextraction of heat in the condenser of the first cycle is performed byexchange of heat with an environmental fluid at ambient temperature. 3.The method as claimed in claim 1, characterized in that, in at least oneof said cycles, the heating of said zeolite adsorbent (Z) to beregenerated is performed by exchange of heat with an environmental fluidat ambient temperature.
 4. The method as claimed in claim 1,characterized in that it comprises the step that consists in performingat least one exchange of heat, preferably at least N-1 exchanges ofheat, each heat exchange being between said zeolite adsorbent (Z)undergoing adsorption in an adsorption/desorption chamber (121) of onecycle and said zeolite adsorbent (Z) undergoing regeneration in anadsorption/desorption chamber (223) of the next cycle.
 5. The method asclaimed in claim 1, characterized in that it comprises the step thatconsists in performing at least one exchange of heat, preferably N-1exchanges of heat, each heat exchange being between the refrigeratingfluid in the evaporator (103, 203) of one cycle and said zeoliteadsorbent (Z) in the adsorption/desorption chamber (220, 320) of thenext cycle undergoing adsorption, in order to cool said zeoliteadsorbent.
 6. The method as claimed in claim 1, characterized in that,in each cycle, there are at least two adsorption/desorption chambers, sothat said adsorption of the refrigerating fluid can be performed in one(121, 221) of said adsorption/desorption chambers while at the same timesaid regeneration of the zeolite adsorbent (Z) is being performed inanother (123, 223) of said adsorption/desorption chambers.
 7. The methodas claimed in claim 6, characterized in that, in each cycle, there areat least three adsorption/desorption chambers so that a step of coolingthe zeolite adsorbent (Z) after regeneration can also be performed atthe same time in yet another (122, 222) of said adsorption/desorptionchambers.
 8. The method as claimed in claim 7, characterized in that itcomprises the step that consists in performing at least one exchange ofheat, preferably N-1 exchanges of heat, each heat exchange being betweenthe refrigerating fluid in the evaporator (103) of one cycle and saidzeolite adsorbent (Z) in the adsorption/desorption chamber (222) of thenext cycle undergoing post-regeneration cooling.
 9. The method asclaimed in claim 1, characterized by the step that consists, in at leastone of said cycles, preferably in each of said cycles, in cooling saidquantity of refrigerating fluid in the vapor phase by exchange of heatwith a source at ambient temperature before said quantity ofrefrigerating fluid is reintroduced into the condenser.
 10. The methodas claimed in claim 1, characterized in that, in at least one of saidcycles, the first pressure (P₂) in said condenser (101, 201, 301, 401,501, 601) is lower than 3 bar, preferably close to normal pressure. 11.The method as claimed in claim 1, characterized in that, in at least oneof said cycles, the maximum pressure is lower than 5 bar, preferablyclose to normal pressure.
 12. The method as claimed in claim 1,characterized in that, in at least one of said cycles, saidrefrigerating fluid in the liquid phase is introduced in atomized forminto the evaporator (103, 203, 303, 403, 503, 603).
 13. The method asclaimed in claim 1, characterized in that the partial pressure of air ineach cycle is less than about 1 kPa, preferably less than about 0.1 kPa.14. The method as claimed in claim 1, characterized in that therefrigerating fluid in the first cycle (100, 400) is chosen from thegroup consisting of water, alcohols and mixtures thereof.
 15. The methodas claimed in claim 14, characterized in that the refrigerating fluid inthe second cycle (200, 500) is chosen from the group consisting ofbutane, butadiene, propadiene, propane and mixtures thereof.
 16. Themethod as claimed in claim 15, characterized in that it comprises athird cycle (300, 600) with a refrigerating fluid chosen from the groupconsisting of ethane, carbon dioxide, nitrous oxide and mixturesthereof.
 17. The method as claimed in claim 16, characterized in that itcomprises a fourth cycle with a refrigerating fluid chosen from thegroup consisting of methane, krypton and mixtures thereof.
 18. Themethod as claimed in claim 17, characterized in that it comprises afifth cycle with a refrigerating fluid chosen from the group consistingof neon, oxygen, helium, nitrogen, argon, carbon monoxide and mixturesthereof.
 19. The method as claimed in claim 1, characterized in that, inat least one of said cycles, said refrigerating fluid has a latent heatof vaporization higher than 300 kJ/kg, preferably greater than or equalto about 450 kJ/kg.
 20. The method as claimed in claim 1, characterizedin that, in at least one of said cycles, the vaporization temperature(T₁) in the evaporator is above the triple point of said refrigeratingfluid.
 21. The method as claimed in claim 1, characterized in that saidproduct (P) is initially in the vapor phase and in that said product iscooled until it liquefies.
 22. The method as claimed in claim 21,characterized in that said product (P) is a gas used as a fuel or as apolymerizable raw material.
 23. The method as claimed in claim 1,characterized in that said product (P) is a gas for use as a rawmaterial that is cooled or liquefied to between −80° C. and −220° C. 24.A device for implementing the method as claimed in claim 1, comprising Nordered cooling stages (100, 200, 300, 400, 500, 600) performed undervacuum, N being an integer greater than 1, each stage comprising: acondenser (101, 201, 301, 401, 501, 601) which contains a refrigeratingfluid in a liquid phase, an evaporator (103, 203, 303, 403, 503, 603)connected to said condenser by a pipe (104, 204, 304, 404, 504, 604), atleast one adsorption/desorption chamber (120, 220, 320, 421-423,521-523, 621-623) containing a zeolite adsorbent (Z) and connected tosaid evaporator via an upstream valve (130, 230, 330), a pipe (160, 260,360, 460, 560, 660) equipped with a downstream valve (150, 250, 350) forreturning said refrigerating fluid from said adsorption/desorptionchamber to said condenser, a heating means (140, 240, 243, 340) in saidor each adsorption/desorption chamber able to heat said zeoliteadsorbent to a regeneration temperature, said device comprising N-1 heatexchangers (280, 380, 501, 601) each arranged in such a way as toexchange heat between the refrigerating fluid in the evaporator (103,203, 403, 503) of one stage and the refrigerating fluid in the condenser(201, 301, 501, 601) of the next stage in the order of the cycles inorder to cool this fluid, and a final heat exchanger (80, 701) arrangedin such a way as to exchange heat between a product (P) that is to becooled and the refrigerating fluid in at least the evaporator of thelast stage (303, 603).
 25. The device as claimed in claim 24,characterized in that it comprises a heat exchanger (126, 480) arrangedin such a way as to exchange heat between the refrigerating fluid in thecondenser (101, 401) of the first stage and an environmental fluid atambient temperature.
 26. The device as claimed in claim 24,characterized in that it comprises, by way of heating means for heatingat least one of said adsorption/desorption chambers (521-523, 621-623),a heat exchanger (540, 640) arranged in such a way as to exchange heatbetween said zeolite adsorbent (Z) undergoing adsorption and anenvironmental fluid at ambient temperature.
 27. The device as claimed inclaim 24, characterized in that it comprises, in at least one of saidstages, a liquid-atomization device (435, 535, 635) arranged in such away as to atomize the refrigerating fluid in the liquid phase as it isintroduced into the evaporator (403, 503, 603).
 28. The device asclaimed in claim 24, characterized in that, in at least one of saidstages, a cooling chamber (216, 316) for cooling the refrigerating fluidis arranged between said or each adsorption/desorption chamber (220,320) and said condenser (201, 301) and is in thermal contact with asource of heat at ambient temperature.
 29. The device as claimed inclaim 24, characterized in that it comprises, by way of heating meansfor heating said adsorption/desorption chambers, at least one heatexchanger (290), preferably at least N-1 heat exchangers, each arrangedin such a way as to exchange heat between said zeolite adsorbent (Z)undergoing adsorption in said or one of said adsorption/desorptionchamber(s) (121) of one stage and said zeolite adsorbent (Z) undergoingregeneration in said or one of said adsorption/desorption chamber(s)(223) of the next stage.
 30. The device as claimed in claim 24,characterized in that it comprises, by way of cooling means for coolingsaid adsorption/desorption chambers, at least N-1 heat exchangers (280;380) each arranged in such a way as to exchange heat between therefrigerating fluid in the evaporator (103; 203) of one stage and saidzeolite adsorbent (Z) in said or each adsorption/desorption chamber(221, 222, 223; 320) of the next stage.
 31. The device as claimed inclaim 24, characterized in that each stage comprises at least twoadsorption/desorption chambers (121, 122, 123) each connected to saidevaporator (103) via a respective upstream valve (131, 132, 133) and tosaid condenser (101) via a respective downstream valve (151, 152, 153).32. The device as claimed in claim 31, characterized in that itcomprises a means of controlling said valves (105) which is programmedto open and close said upstream and downstream valves in a cycle ofconcurrent operations, in which each chamber (121, 122, 123) performs insuccession an adsorption step, for which the upstream valve (131) isopen and the downstream valve (151) is closed, a regeneration ordesorption step for which the downstream valve (153) is open and theupstream valve (133) is closed, and a post-regeneration cooling step forwhich the downstream valve (152) and the upstream valve (132) areclosed.
 33. The device as claimed in claim 24, characterized in that itis associated with a chamber (1) containing said product that is to becooled, said final heat exchanger (26) being supported within saidchamber in order to exchange heat between the refrigerating fluid in theevaporator (303) of the last stage and the product (P) in the liquid orvapor phase contained in said chamber.
 34. A methane tanker equippedwith a storage tank (1) for liquefied gas (P), with which a device asclaimed in claim 33 is associated by way of a refrigeratingre-liquefaction unit.
 35. A gas-liquefaction plant comprising a coolingchamber (1) for cooling the gas (P) that is to be liquefied, whichchamber is associated with a device as claimed in claim 33.